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ReflectionOxford AQA IGCSE Physics: Revision notes

Section 1

What is the law of reflection?

The law of reflection states that the angle of incidence equals the angle of reflection.

Both angles are measured from the normal — an imaginary line drawn perpendicular (at 90°) to the reflecting surface at the point where the ray strikes it.

  • The angle of incidence is the angle between the incoming ray and the normal
  • The angle of reflection is the angle between the reflected ray and the normal
Key termslaw of reflectionnormal
Common mistake

Angles of incidence and reflection are always measured from the normal, never from the surface itself — a very common source of lost marks.

Section 2

What kind of image does a plane mirror form?

A plane (flat) mirror forms an image that is:

  • Virtual — the light rays only appear to come from behind the mirror; they do not actually meet there, so the image cannot be projected onto a screen
  • Upright — the same way up as the object
  • Laterally inverted — left and right are swapped (this is why text appears backwards in a mirror)

The image is also the same size as the object, and appears to be the same distance behind the mirror as the object is in front of it.

Key termsvirtual imagelaterally inverted
Think of it like this

Raise your right hand in front of a mirror — your reflection appears to raise its left hand. That swap is lateral inversion.

Section 3

How do you construct a ray diagram for reflection?

To construct a ray diagram showing reflection from a plane mirror:

  1. Draw the mirror as a straight line
  2. Draw the normal as a dashed line, perpendicular to the mirror at the point where the ray hits it
  3. Draw the incident ray travelling towards the point of incidence, with an arrow showing the direction of travel
  4. Measure the angle of incidence between the incident ray and the normal
  5. Draw the reflected ray on the other side of the normal, at an equal angle (the angle of reflection), with an arrow showing the direction of travel

Ray diagrams are used to represent the changing path of reflected rays and to locate images formed by mirrors.

Exam tip

Always use a ruler and protractor for ray diagrams in practical work, and always draw the normal as a dashed line to distinguish it from the actual light rays.

Must Know

  • Law of reflection: angle of incidence equals angle of reflection
  • Both angles are measured from the normal, a line perpendicular to the reflecting surface at the point of incidence
  • A plane mirror forms an image that is virtual, upright and laterally inverted
  • The image in a plane mirror is the same size as the object and appears as far behind the mirror as the object is in front
  • Ray diagrams show the incident ray, normal, and reflected ray, with equal angles either side of the normal

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